The topography of a country has a lot to do with a country's culture. What they wear, the houses they live in, what they eat, and exposure to religion all play a part in its cultural development.
It's easy to see why Japan has a separate culture. It's located several hundred miles from the mainland and therefore somewhat removed from the countries of China and Korea. Because of the great distance to travel to get to Japan and the relative difficulties of such a journey, it couldn't be colonized or conquered by any other country or culture.
In the case of China and Korea: there are high mountains between them that provide a natural barrier which could explain the differences in their two cultures.
Cuz they eyes too small BRRRRKAAYY